The Fischer-Tropsch process was developed in the past century and the industrial production was opened up at once. Historically the synthesis was realized in fixed-bed reactors and then in reactors with ever more complicated designs due to the requirements of an increase in the catalyst efficiency and a need of solution of rising problems with heat removal.
The Fischer-Tropsch synthesis is carried out at high pressure over catalyst based on a metal selected from Group VIII of the Mendeleev's Periodic Table of the Elements; the process is exothermic.
It is commonly required that a catalytic bed in the Fischer-Tropsch process is characterized by high concentration of catalytically active component in the reaction volume; by small size of the catalyst particles; by high heat conductivity of the catalyst bed; by high surface area of the gas-liquid interphase; and finally by the flow regime close to ideal plug-in. All these requirements define a strong interrelation between selection of the catalyst and selection of the reactor design.
It is well known that the heterogeneous exothermic processes can be carried out technologically in the fluidized reactors, the slurry phase reactors and in the fixed-bed reactors.
In virtue of a simple and detailed working out of the main structural solution the fixed-bed reactors are the most commonly encountered systems in the area of the catalytic technologies. Inside the reaction tubes such reactors have a heterogeneous system that consists of at least two phases: solid particles of the catalyst and spaces in between where the reaction mixture flows in form of gas and/or liquid. Simultaneously the chemical conversions on the catalyst surface and the following physical processes take place in the reactor: the reaction component and product transfer in the bed, heat transfer and gas flow etc.
The formation of the optimum temperature range in the catalyst bed is one of the main problems faced by specialists during the development of the catalytic tubular reactors. The optimal temperature range promotes an improvement of heat and mass transfer in each catalyst pellet.
There is another problem which comes out only in the high-efficiency reactors; this refers to low rate of withdrawal of the liquid products from reaction tubes. An accumulation of the products in the bottom part of the reaction tubes results in a growth of the pressure difference in reactor, a flooding of the reaction tubes and a complete halt of the reaction.
Shell (housing) and tube reactor aka tubular reactor for chemical processes with fixed bed of a catalyst is disclosed (see Laschinskiy A. A., Tolchinskiy A. R., Principles of design and calculation of chemical apparatus, L., Engineering industry, 1970, 752 p.). In such reactor the heat transfer between the reaction mixture and cooling medium is realized through the reactor wall. The catalyst is packed into the tubes of the small diameter (2-8 cm); a coolant (e.g. high pressure steam) circulates in the intertubular space. The important advantage of the tubular reactors is favorable terms for heat removal from the catalyst because the ratio of the cooling surface to the catalyst volume is well over than in other apparatuses.
U.S. 2,240,481 discloses the modification of the shell and tube reactor wherein a boiling liquid (its temperature is regulated by pressure) is used for cooling the tubes with fixed bed of the catalyst. Excess of the generated heat goes to the inner heat of liquid evaporation.
U.S. Pat. No. 7,012,103 relates to a process for producing synthetic liquid hydrocarbons by converting synthesis gas (syngas) under the Fischer-Tropsch reaction on a fixed catalyst bed in a vertical shell and tube reactor and a reactor for Fischer-Tropsch synthesis comprising tubes with catalyst (at least 100, each tube has a height between 2 and 5 meters) in the shell (housing), syngas and coolant inlets, product and steam outlets.
The main drawback of the disclosed process and reactor is low rate of withdrawal of the liquid products from the reaction tubes. An accumulation of the products in the bottom part of the reaction tubes results in a growth of the pressure difference in the reactor, a flooding of the reaction tubes and a complete halt of the reaction.
The commonly accepted methods for overcoming the problem comprise either limitation of productivity by limiting the feedgas flow (this indeed allows possibility for the produced liquid products to flow down) or an increase in temperature beyond an optimum level. The latter method results in poor product quality due to low reaction selectivity for the target products.